A pozzolan is a material, which when combined with calcium hydroxide, exhibits cementitious properties in the presence of water. Pozzolanic ash can be used as a component of blended cements, if it meets certain quality specifications or is sufficiently upgraded. Cement clinker components release calcium hydroxide during hydration necessary to convert pozzolan to cementitious material. Some ash is cementitious on its own; e.g. subbituminous or most lignite coal ash.
The technical benefits gained from the addition of such materials, including pozzolans or coal derived cementitious ash, especially very fine materials, include reduced use of Portland cement per unit volume of concrete, and increased strength and reduced water permeability of the concrete product. For example, it is generally accepted that fly ash with fine particles will participate more rapidly and completely in the cementitious reactions of blended cement concrete than fly ash with coarse particles. Sieved ash gave higher compressive strengths in concrete with lower water demand than the original coarse ash (“Pulverised Coal Ash—Requirements for Utilization”, IEA Coal Research, 1996). Economic benefits arise from the reduction in the amount of Portland cement used, and from the higher quality concrete produced. Environmental benefits arise from reduced energy use (e.g. reduced greenhouse gas emissions) in cement manufacture. Use of waste materials such as high carbon bituminous coal fly ash as pozzolan provides additional economic and environmental benefits by reducing the amount of material that must be disposed of in landfills, etc.
The value of bituminous coal fly ash as pozzolan is limited by its carbon content which adsorbs costly organic concrete additives such as air entrainers, as well as affecting the colour of the concrete. The value of bituminous coal fly ash as a pozzolan is enhanced by reducing its particle size, which increases the surface area and reactivity of silica and siliceous materials towards the free calcium hydroxide generated by hydration of Portland cement as well as reducing water porosity and increasing compressive strength of the resulting concrete. Particle size reduction also results in carbon liberation (breaking apart carbon and siliceous ash), thus facilitating separation of the carbon from mineral ash.
It is well known that high levels of ammonia in cementitious materials and pozzolans are detrimental to blended cement and concrete applications since alkaline cement (e.g. Portland) becomes highly alkaline on addition of water which releases ammonia in the cementitious or pozzolanic material to undesirable levels. It would be desirable to reduce the ammonia level of cementitious materials and pozzolans including fly ash.
Because of the foregoing problems, individuals and companies in the field have endeavoured to enhance the quality of pozzolanic ash products. For instance, it is well known that particle size reduction enhances the value of pozzolanic or cementitious combustion ash for both blended cement applications and cement kiln raw material input. For example, and more specifically, particle size reduction of ash enhances early strength of blended cements. ASTM Method C618 requires that the amount of ash retained on a 45 micron sieve be less than 34% of the ash input to blended cement. Some ashes do meet this specification without grinding.
Minkara and Heavilon (U.S. Pat. No. 5,840,179) describe the use of ultrasonic energy to condition a fly ash-water slurry prior to carbon removal by froth flotation. The patent describes a “conditioning” treatment in which a fly ash-water slurry is subjected to high intensity ultrasound treatment. A “conditioning”, or surface active, agent selectively wets the carbon surfaces with oleophilic material. Carbon-rich particles are recovered after conditioning by flotation, providing an upgraded fly ash pozzolan product. The purpose of this invention is to enhance fly ash as a pozzolan to reduce its carbon content and/or increase fineness (reduce particle size) and increase surface area. This patent indicates that prior art mechanical conditioning of ash before carbon flotation is typically one-half hour or more and is believed to reduce or degrade the pozzolanic quality of the fly ash pozzolan (see column 2, lines 11 to 43). This invention achieved median particles size reduction of 31% and 59% for American and Columbian bituminous coal fly ash respectively (see column 5, lines 1 to 4). Finally, this invention achieved a maximum increase in particle surface area of 65% (see table in columns 9 and 10).
In spite of useful fly ash pozzolan enhancement, the process of Minkara and Heavilon suffers from several disadvantages, including high cost and limited lifetime of ultrasound transducers and horns; inefficient use of energy; and potential for slurry short-circuiting around the equipment, which are effectively point sources.
Gray et al (U.S. Pat. No. 6,126,014) describe an agglomeration/flotation process for concentration of carbon-bearing material in fly ash. Although this method provides high carbon recovery at moderate purity, residual ash composition and pozzolanic properties are not specified. Disadvantages of the Gray process include use of expensive light hydrocarbon solvent with limited recovery; explosion hazard in the presence of air; and limited capability to liberate carbon particles attached to mineral ash.
One important parameter that determines the quality of combustion ash with respect to pozzolanic applications is the size of the ash particles, which determines the specific surface area. Processes and devices, such as Pearl mills, are known in the art that are capable of producing ash particle sizes at or below 5 μm. However, such pulverization processes require hours, which significantly detracts from their utility and, in some cases, from the quality of the product. A process having a pulverization of time below 10 minutes, preferably below 5 minutes, more preferably at or below 3 minutes and even as low as 1 minute and yet producing ash particles with a median size of less than 20 μm and surface areas of 0.9 m2/g or higher, would be a very significant advance in the field.
Use of high silica materials such as clay, shale, sand and combustion ash as raw materials to cement kilns in cement clinker production is well known and widely practiced. U.S. Pat. No. 6,391,105 to Oates et al. indicates that the yield of cement clinker recovered from a kiln is enhanced by feeding fly ash into the hot clinker and that, in general, smaller particles more readily partially fuse into the hot clinker at a given temperature and exposure time. The prior art has not revealed how smaller combustion ash particles can be generated rapidly.
Accordingly, there exists a need for a practical, economical, large-scale process that can quickly upgrade combustion ash by increasing the specific surface area and decreasing the particle size. Such a process that includes or contributes to the reduction of ammonia, sulphur, and/or carbon content of the ash would have even greater utility. Such a process should achieve these enhancements while reducing pulverization time in order to minimize equipment costs, operating costs, and pozzolanic properties of the combustion ash, or its value as a cement kiln raw material. Ideally, the process would be amenable to being performed with equipment that is readily available, without the necessity of constructing equipment de novo.